The specific aims are to analyze interactions among the embryonic neural tube and adjacent chordamesoderm during normal and abnormal development in mutant mice which exhibit neural dysraphism and in rhesus monkey embryos in which neural tube malformations have been induced by treatment with triamcinolone acetonide. A variety of techniques will be used at the electron microscopic level, including acid phosphatase cytochemistry, ruthenium red and tannic acid histochemistry, and alcian blue at critical electrolyte concentrations with differential enzyme digestion. Tritiated thymidine autoradiography at the light microscopic level will also be used. This project will help to contribute to an understanding of the etiology of specific human neurological disorders such as exencephaly, meningocele, anencephaly, and spina bifida.